SQL ESCAPE statement in the LIKE-query


Example

If you implement a text-search as LIKE-query, you usually do it like this:

SELECT * 
FROM T_Whatever 
WHERE SomeField LIKE CONCAT('%', @in_SearchText, '%') 

However, (apart from the fact that you shouldn't necessarely use LIKE when you can use fulltext-search) this creates a problem when somebody inputs text like "50%" or "a_b".

So (instead of switching to fulltext-search), you can solve that problem using the LIKE-escape statement:

SELECT * 
FROM T_Whatever 
WHERE SomeField LIKE CONCAT('%', @in_SearchText, '%') ESCAPE '\'

That means \ will now be treated as ESCAPE character. This means, you can now just prepend \ to every character in the string you search, and the results will start to be correct, even when the user enters a special character like % or _.

e.g.

string stringToSearch = "abc_def 50%";
string newString = "";
foreach(char c in stringToSearch) 
     newString += @"\" + c;
 
sqlCmd.Parameters.Add("@in_SearchText", newString); 
// instead of sqlCmd.Parameters.Add("@in_SearchText", stringToSearch);

Note: The above algorithm is for demonstration purposes only. It will not work in cases where 1 grapheme consists out of several characters (utf-8). e.g. string stringToSearch = "Les Mise\u0301rables"; You'll need to do this for each grapheme, not for each character. You should not use the above algorithm if you're dealing with Asian/East-Asian/South-Asian languages. Or rather, if you want correct code to begin with, you should just do that for each graphemeCluster.

See also ReverseString, a C# interview-question